ˌunder-the-ˈcounter adjective informal
under-the-counter goods are bought or sold secretly, especially because they are illegal

[TahlilGaran] Dictionary of Contemporary English ▲

under the counter
if something is bought or sold under the counter it is bought or sold secretly or in a way that is not legal.
Many of his books are banned and only available under the counter.

[TahlilGaran] English Idioms Dictionary ▲

under the counter
secretly bought or sold
The drugs are being sold under the counter although the government has not given its approval.

[TahlilGaran] English Idioms Dictionary ▲

under the counter
adv. phr., informal Secretly (bought or sold).
That book has been banned, but there is one place you can get it under the counter.
The liquor dealer was arrested for selling beer under the counter to teenagers.
■ Also used like an adjective, with hyphens.
During World War II, some stores kept scarce things hidden for under-the-counter-sales to good customers.

[TahlilGaran] English Idioms Dictionary ▲